Corrosion and oxidation protective coatings of this type are known and are used in particular in parts of turbines or aircraft propulsion engines, as well as in combustion chambers. MCrAlY coatings are used as hot-corrosion protective coatings, as are described, for example, in U.S. Pat. No. 4,080,486, EP-B 1-0486489, and U.S. Pat. No. 4,585,481. In addition, these MCrAlY coatings may be used as an adhesion promoter or as an adhesive layer between the metallic substrate to which the protective coating is applied and a ceramic cover layer. The protective coating is applied in particular through thermal spraying methods, e.g., flame spraying, high-speed flame spraying, detonation spraying, plasma spraying, arc spraying, laser spraying, or molten bath spraying. The qualitatively best results have been achieved by the low-pressure plasma spraying method (LPPS), since closed and dense coatings arise here. Other plasma spraying methods, such as atmospheric plasma spraying (APS), achieve poorer results. Thus, the APS method is the most cost-effective method, but the resulting spray coatings have a very high number of pore inclusions and, in particular, connected oxide inclusions and oxide streaks. For this reason, the coatings produced using the APS method have the lowest quality in regard to their hot gas corrosion resistance at temperatures in the range of 1000° C. in comparison to other plasma spraying methods. In particular, the oxide and nitride inclusions increasingly occurring in the APS method and the connection of these inclusions into spatial networks represent ideal migration paths for, among other things, O2 at high temperatures, because of which these coatings are relatively susceptible to corrosion.
However, the APS method has decisive advantages. In particular, it is a cost-effective coating method in comparison to the other plasma spraying methods. Furthermore, there is a large possibility for variation of the composition of the metallic material, in particular of the powder composition. In addition, besides the chemical composition, the particle sizes and the coating gradation may be varied in different layers. Furthermore, it is possible for defined surface roughnesses to be set for clamping ceramic cover layers to the protective coating.
Different methods are known for improving the properties of APS-sprayed coatings. Thus, DE-A-2414992 describes a method for improving the high-temperature corrosion resistance of a nickel-based or cobalt-based superalloy body. In this case, the superalloy body is first coated using physical vapor deposition with a composition which is essentially composed of chromium, aluminum, and a part that is selected from yttrium and the rare earth elements, and at least one element which is selected from the group including iron, cobalt, and nickel. Subsequently, the coated body is aluminized using chemical vapor deposition to elevate the corrosion resistance of the body. Through the aluminizing coating, grain boundaries of the first coating, which are oriented in a direction perpendicular to the deposition plane, are to be closed. DE-T2-69536781 also describes a method for improving the oxidation resistance of a platinum-modified aluminite coating produced on a nickel-based superalloy substrate. In this case, a platinum layer is first provided on the substrate. The platinum layer is subsequently aluminized.